Method of manufacturing semiconductor device including forming protective film within recess in substrate

ABSTRACT

An n-type layer ( 3 ) is formed by implanting an n-type impurity in a back surface of a Si substrate ( 1 ). A recess ( 4 ) is formed in the back surface of the Si substrate ( 1 ). After forming the n-type layer ( 3 ), an oxide film ( 5 ) is formed on the back surface and in the recess ( 4 ). The oxide film ( 5 ) on the back surface is removed while the oxide film ( 5 ) in the recess ( 4 ) is left. After removing the oxide film ( 5 ), an Al—Si film ( 6 ) is formed on the back surface. A metal electrode ( 7 ) is formed on the Al—Si film ( 6 ). The oxide film ( 5 ) in the recess ( 4 ) prevents Al from diffusing from the Al—Si film ( 6 ) into the Si substrate ( 1 ) through the recess ( 4 ).

FIELD

The present invention relates to a method of manufacturing asemiconductor device wherein a metal electrode is formed on a backsurface of a Si substrate.

BACKGROUND

In a power semiconductor device for driving a large current, a metalelectrode k formed on a back surface of a Si substrate (see, forexample, PTL 1). An Al—Si film is formed between the Si substrate andthe metal electrode for the purpose, of inhibiting separationtherebetween.

CITATION LIST Patent Literature

PTL 1: JP 2006-074024 A

SUMMARY Technical Problem

A recess is formed in the back surface of the Si substrate due tomechanical stress at the time of wafer manufacturing. Al diffuses fromthe Al—Si film into the Si substrate through this recess, therebyforming a p-type layer. There is a problem that a depletion layerreaches this p-type layer to cause a leak current and a reduction inwithstand voltage of the device.

The present invention has been achieved to solve the above-describedproblem, and an object of the present invention is to provide a methodof manufacturing a semiconductor device capable of inhibiting a leakcurrent and a reduction in withstand voltage even in a case where arecess exists in a back surface of a Si substrate.

Solution to Problem

A method of manufacturing a semiconductor device according to thepresent invention includes: forming an n-type layer by implanting ann-type impurity in a back surface of a Si substrate wherein a recess isformed in the back surface; after forming the n-type layer, forming aprotective film on the back surface and in the recess; removing theprotective film on the hack surface while the protective film in therecess is left; after removing the protective film, forming an Al—Sifilm on the hack surface; and forming a metal electrode on the Al—Sifilm, wherein the protective film in the recess prevents Al fromdiffusing from the Al—Si film into the Si substrate through the recess.

Advantageous Effects of Invention

In the present invention, the protective film in the recess prevents Alfrom diffusing from the Al—Si film into the Si substrate through therecess. Thus, although the recess exists in the back surface of the Sisubstrate, a teak current and a reduction in withstand voltage can beinhibited.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 4 are sectional views showing the method of manufacturing asemiconductor device according to Embodiment 1 of the present invention.

FIG. 5 is a sectional view showing a method of manufacturing asemiconductor device according to a comparative example.

FIG. 6 is a sectional view showing a method of manufacturing asemiconductor device according to Embodiment 2 of the present invention.

FIG. 7 is a sectional view showing a method of manufacturing asemiconductor device according to Embodiment 3 of the present invention.

FIG. 8 is a sectional view showing a method of manufacturing asemiconductor device according to Embodiment 4 of the present invention.

FIG. 9 is a sectional view showing a method of manufacturing asemiconductor device according to Embodiment 5 of the present invention.

FIGS. 10 and 11 are sectional views showing a method of manufacturing asemiconductor device according to Embodiment 6 of the present invention.

FIG. 12 is a sectional view showing a modified example of the method ofmanufacturing a semiconductor device according to Embodiment 6 of thepresent invention.

DESCRIPTION OF EMBODIMENTS

A method of manufacturing a semiconductor device according to theembodiments of the present invention will be described with reference tothe drawings. The same components will be denoted by the same symbols,and the repeated description thereof may be omitted.

Embodiment 1

A method of manufacturing a semiconductor device according to Embodiment1 of the present invention will be described with reference to thedrawings. FIGS. 1 to 4 are sectional views showing the method ofmanufacturing a semiconductor device according to Embodiment 1 of thepresent invention.

First, as shown in FIG. 1, a p-type layer 2 is formed by implanting ap-type impurity into a front surface of an n⁻-type Si substrate 1, andan n-type layer 3 is formed by implanting an n-type impurity in a backsurface of the substrate 1. A recess 4 (flaw) is formed in the backsurface of the Si substrate 1. Subsequently, as shown in FIG. 2, anoxide film 5 is formed on the back surface and in the recess 4.Subsequently, as shown in FIG. 3, the oxide film 5 on the back surfaceis removed by sputter-etching while the oxide film 5 in the recess 4 isleft. It is necessary that at this time part of the oxide film 5 be leftat least in an innermost portion of the recess 4. Subsequently, as shownin FIG. 4, an Al—Si film 6 is formed on the back surface. Subsequently,a metal electrode 7 is formed on the Al—Si film 6. The metal electrode 7is a multilayer electrode.

The advantageous effect of the present embodiment will be describedthrough comparison with a comparative example. FIG. 5 is a sectionalview showing a method of manufacturing a semiconductor device accordingto a comparative example. In the comparative example, the Al—Si film 6is in direct contact with the Si substrate 1 in an innermost portion ofthe recess 4. When a reverse voltage is applied to the device,therefore. Al diffuses from the Al—Si film 6 into the Si substrate 1through the recess 4 to form a p-type layer. Because a depletion layerreaches this p-type layer, a leak current is generated and the withstandvoltage of the device is reduced.

On the other hand, in the present embodiment, the oxide film 5 in therecess 4 functions as a protective film for preventing Al from diffusingfrom the Al—Si film 6 into the Si substrate 1 through the recess 4.Thus, although the recess 4 exists in the back surface of the Sisubstrate 1, a leak current and a reduction in withstand voltage can beinhibited. Also, separation between the Si substrate 1 and the metalelectrode 7 can be inhibited with the Al—Si film 6.

Embodiment 2

FIG. 6 is a sectional view showing a method of manufacturing asemiconductor device according to Embodiment 2 of the present invention.In the present embodiment, an n-type doped polysilicon 8 is formed as aprotective film in place of the oxide film 5 in Embodiment 1. In thiscase, the n-type doped polysilicon S in the recess 4 prevents Al fromdiffusing the Al—Si film 6 into the Si substrate 1 through the recess 4,thus obtaining the same advantage as in Embodiment 1. Since the n-dopedpolysilicon 8 is of the n-type, there is a margin for Al that activatesthe Si substrate 1 into the p-type and energization can be effected eventhrough the recess 4.

Embodiment 3

FIG. 7 is a sectional view showing a method of manufacturing asemiconductor device according to Embodiment 3 of the present invention.In the present embodiment, a phosphosilicate glass 9 is formed as aprotective film in place of the oxide film 5 in Embodiment 1. Also inthis case, the same advantage as that in Embodiment 1 can be obtained.Since the phosphosilicate glass 9 is of the n-type, there is a marginfor Al that activates the Si substrate 1 into the p-type andenergization can be effected even through the recess 4.

Embodiment 4

FIG. 8 is a sectional view showing a method of manufacturing asemiconductor device according to Embodiment 4 of the present invention.In the present embodiment, a metal simple substance 10 (titanium) notsilicidized is formed as a protective film in place of the oxide film 5in Embodiment 1. Because the metal simple substance 10 in the recess 4functions as a barrier metal for preventing diffusion of Al, the sameadvantage as that in Embodiment 1 can be obtained. The metal simplesubstance 10 has a good ohmic characteristic with respect to the n-type.

Embodiment 5

FIG. 9 is a sectional view showing a method of manufacturing asemiconductor device according to Embodiment 5 of the present invention.In the present embodiment, a metal film (titanium, cobalt, tungsten ormolybdenum) is formed on the back surface and in the recess 4 after then-type layer 3 is formed. Thereafter, metal suicide 11 (titaniumsuicide, cobalt suicide, tungsten suicide or molybdenum suicide) isformed by performing annealing. Subsequently, the metal suicide 11 onthe back surface is removed by sputter-etching while the metal silicide11 in the recess 4 is left. Al—Si film 6 and a metal electrode 7 arethereafter formed, as are those in Embodiment 1.

The metal silicide 11 in the recess 4 functions as a protective film forpreventing Al from diffusing from the Al—Si film 6 into the Si substrate1 through the recess 4. The same advantage as that in Embodiment 1 cantherefore be obtained. The metal suicide 11 is higher in ohmiccharacteristic with respect to the n-type layer 3 than the metal simplesubstance 10 and functions as a barrier metal for preventing diffusionof Al.

Embodiment 6

FIGS. 10 and 11 are sectional views showing a method of manufacturing asemiconductor device according to Embodiment 6 of the present invention.First, in the same way as in Embodiment 1, and as shown in FIG. 1, thep-type layer 2 is formed by implanting a p-type impurity into a frontsurface of the n⁻-type Si substrate 1, and the n-type layer 3 is formedby implanting an n-type impurity in the back surface of the substrate 1.The recess 4 is formed in the back surface of the Si substrate 1.Subsequently, as shown in FIG. 10, the back surface is recrystallized bylaser annealing the back surface while the recess 4 is left, therebyre-forming the n-type layer 3. Subsequently, as shown in FIG. 11, theAl—Si film 6 is formed on the back surface. The metal electrode 7 isformed on the Al—Si film 6.

Prevention of diffusion of Al from the Al—Si film 6 into the Sisubstrate 1 through the recess 4 is thus enabled. The same advantage asthat in Embodiment 1 can therefore be obtained. Also, since the n-typelayer 3 can be re-formed, a change in characteristic can be inhibited.While the method of eliminating the recess 4 by melting the back surfaceby laser application requires a long processing time, the recess 4 isleft in the present embodiment to enable shortening the laser annealingprocessing time and, hence, reducing the manufacturing cost.

FIG. 12 is a sectional view showing a modified example of the method ofmanufacturing a semiconductor device according to Embodiment 6 of thepresent invention. Laser annealing is selectively performed only on therecess 4. Recrystallization of a portion including and around the recess4 can be performed in this way and the advantage of Embodiment 6 cantherefore be obtained. Since laser annealing is selectively performed,the laser annealing processing time can be further reduced and areduction in manufacturing cost can therefore be achieved.

REFERENCE SIGNS LIST

1 Si substrate; 3 n-type layer; 4 recess; 5 oxide film; 6 Al—Si film; 7metal electrode; 8 n-type doped polysilicon; 9 phosphosilicate glass; 10metal simple substance; 11 metal suicide

The invention claimed is:
 1. A method of manufacturing a semiconductordevice comprising: forming an n-type layer by implanting an n-typeimpurity in a back surface of a Si substrate wherein a recess is formedin the back surface; after forming the n-type layer, forming aprotective film directly on the back surface and in the recess; removingthe protective film on the back surface while the protective film in therecess is left; after removing the protective film, forming an Al—Sifilm directly on both the back surface and a surface of the protectivefilm, wherein the surface of the protective film directly on the Al—Sifilm is directly under the recess; and forming a metal electrode on theAl—Si film, wherein the protective film in the recess prevents Al fromdiffusing from the Al—Si film into the Si substrate through the recess.2. The method of manufacturing a semiconductor device according to claim1, wherein the protective film is an oxide film.
 3. The method ofmanufacturing a semiconductor device according to claim 1, wherein theprotective film is an n-type doped polysilicon or a phosphosilicateglass.
 4. The method of manufacturing a semiconductor device accordingto claim 1, wherein the protective film is a metal simple substance. 5.The method of manufacturing a semiconductor device according to claim 1,wherein the protective film is metal silicide.
 6. The method ofmanufacturing a semiconductor device according to claim 1, wherein therecess is a flaw.
 7. The method of manufacturing a semiconductor deviceaccording to claim 1, wherein a p-type layer is formed by implanting ap-type impurity into a front surface of the Si substrate.